The present invention relates to the improvement of an aperture antenna, in particular, relates to such an antenna with the improved crosspolarization discrimination. The present antenna can be utilized for a wireless communication system utilizing two polarizations, like a horizontally polarized wave, and a vertically polarized wave.
In a wireless communication system, two orthogonally polarized waves are frequently used for the efficient use of the limited frequency band. In this case, the system quality depends upon the interference between these two polarizations. The interference is increased when it rains, since the orientation polarization rotates by the rain drops and the orthogonality of the polarization is degraded. The other case of increasing the interference is when the fading occurs in the transmission route. In this case, the route of electro-magnetic wave from the transmitting antenna to the receiving antenna becomes multipath. By the difference of each path length of multipath and the characteristics of receiving antenna for the direction of multipath, the interference is increased.
FIG. 1 shows a prior aperture antenna which has been utilized in a microwave band. In the figure, the reference numeral 1 is a main reflector, 2 is a sub-reflector, 3 is a primary radiator which is implemented by a horn structure, 4a and 4b show the direction of the received electric wave, and 5 shows the center axis of the antenna beam. The numeral 6 is an aperture of an antenna, and 7 is the path of the electric wave from the horn 3 to the aperture 6.
When there is no fading, the direction (4a, 4b) of the received wave coincides with the center axis 5 of the antenna beam. However, when there is fading, the directions of the received wave are separated into .phi..sub.1 and .phi..sub.2 direction due to the multipath of the wave. And it should be noted that the phase of the wave received in one direction (.phi..sub.1) is generally different from that in other direction (.phi..sub.2).
FIGS. 2A and 2B show the antenna radiation characteristics of the amplitude and the phase respectively, where a solid line shows the characteristics of the co-polarization, and a dotted line shows the characteristics of the cross-polarization. As can be seen in FIG. 2A, the ratio of the co-polarization to the cross-polarization, or the discrimination of two waves, is larger than 45 dB, when there is no fading and the angle (.phi.) is zero.
However, when there is fading, the phases of waves coming from .phi..sub.1 and .phi..sub.2 directions differ by 180 degree, so the amplitude of the co-polarization is considerably decreased, since two waves having the similar amplitude and the opposite phase are added with each other.
On the other hand, the amplitude of the cross-polarization is increased at the output of the antenna, since the phase of two cross-polarization become the same. The reason for that is as follows. Two cross-polarization from the direction .phi..sub.1 and .phi..sub.2 differs by 180 degrees in free space, the first cross-polarization from the direction .phi..sub.1 has the phase rotation of 90 degrees at the antenna (see a dotted line in FIG. 2B), and the second cross-polarization from the direction .phi..sub.2 has the phase rotation of -90 degrees at the antenna. Thus, the difference between phase rotations of two cross-polarization at the antenna is 180 degrees. Therefore, two waves having the opposite phases in the free space are rotated by 180 degrees by the antenna, then, the resultant phase between the two waves is 360 degrees which is equal to zero degrees.
As a result, when there is fading, the co-polarization is decreased and the cross-polarization is not decreased, and then, the ratio of the co-polarization to the cross-polarization becomes smaller than 45 dB. Therefore, the interference between co-polarization and the cross-polarization occurs. That interference between the co-polarization and the cross-polarization generates an undesirable problem to a microwave communication system which utilizes two polarization waves. However, there has been no effective proposal for decreasing the interference.